1. The Field of the Invention
The present invention relates to single crystalline, polycrystalline or ceramic scintillation materials having a higher strength due to their particular composition.
2. The Description of the Related Art
Scintillation materials are active media which absorb highly energetic radiation, wherein via numerous of intermediate steps electron-hole pairs are generated. In this case, the activator centers are energized into an excited state. Recombination results in emission of visible light.
Ranges of application are in the field of medicine (imaging and diagnostic), industrial inspection, dosimetry, nuclear medicine and high energy physics as well as “security”, object tracking and exploration.
A known family of scintillator crystals is of the type of sodium iodide (NaI) doped with thallium. This scintillation material, which was discovered in the year 1948 from Robert Hofstadter and which is the basis of modern scintillators, is also today the ultimate in this field, despite 50 years of search for other materials. These crystals have a relatively long scintillation decay time.
A material which is also used is CsI which can be used in pure form or doped with thallium and sodium, respectively, depending on the intended purpose.
A family of scintillator crystals which has had a great development is of the type bismuth germanate. But these crystals have long decay times which limit their use to low counting rates.
A younger scintillator family was developed in the nineties and is of the type of lutetium oxyorthosilicate activated with cerium, LSO (Ce). However, these crystals are very heterogenic and have a very high melting point (about 2200° C.).
The development of new scintillator materials having improved properties is the subject matter of numerous studies, because the requirements for scintillator materials are extremely versatile.
The requirements for detector materials for the detection and conversion of highly energetic radiation (X-ray and gamma radiation) into visible light are diverse:                high energy resolution        fast decay time        stopping power        mechanical requirements for the material        high light yield        radiation stability        homogeneity.        
The higher the energy resolution, the better the quality of the detector. So it is estimated that an energy resolution of about 7% allows obtaining good results. Nevertheless, smaller resolution values are of good interest, because in these cases e.g. the contrast and the quality of images which can be provided by medical apparatuses are improved. This for example allows a more exact and earlier detection of tumours.
A very important parameter is the decay time of the scintillators. In this case, a decay time is desired which is as short as possible so that the operation frequency of the detectors can be increased. In the field of nuclear medicine imaging it allows for example a considerably shortened examination time.
Furthermore, a good stopping power of the detectors is desired. The stopping power characterizes the absorption properties of a material. The stopping power specifies how the energy of a particle decreases during the passage through the material. It is defined as the derivative of the energy with respect to the distance. A high electron density results in high interactions during which the gamma quanta are absorbed and result in scintillation. However, only those gamma quanta which are absorbed can produce light. For scintillator materials of the same thickness the quantum efficiency increases with the stopping power. In TOF-PET (time of flight-positron emission tomography) higher stopping power results in a lower thickness of the scintillator with the same intended quantum efficiency and as a result thereof an increased spatial resolution is achieved.
According to prior art lanthanoid halogenides are produced in exact stoichiometry according to a melt growth method, for example according to the Bridgman method, Czochralski method, vertical gradient freeze (VGF) method, horizontal gradient freeze (HGF) method or gradient solidification method (GSM).
In the Bridgman method an ampoule is used in which the melt and the crystal are contained. The ampoule containing the initially solid growth material is vertically moved upwards in the furnace so that the material melts top-down. At the bottom of the ampoule a monocrystalline seed crystal is contained. After this seed crystal has partly been melted, the ampoule is slowly pulled back so that the crystal grows from the bottom up starting at the seed crystal.
In the Czochralski method in a crucible which already contains the desired melt a slowly rotating metal rod on which the seed crystal is mounted is immersed from above into the melt with its tip. The crystal growing on the seed crystal is pulled upwards out of the melt.
In the so-called vertical gradient freeze method (VGF method) several concentric heating circuits which are superimposed upon each other are arranged around the stationary melting crucible in jacket form. Each of these heating circuits can be separately energized. By slowly decreasing the heat output of each single heating circuit arranged around the crucible wall the temperature can be slowly decreased below the crystallisation point, thus producing a radial temperature gradient along which the crystal growth takes place.
The horizontal gradient freeze method (HGF method) is conducted in an analogous way to the VGF method, only that the construction is tilted by 90°.
In the so-called gradient solidification method (GSM) around a stationary melting crucible annular circumambient heating circuits are slowly moved down and up.
Journal of Crystal Growth 279 (2005), pages 390 to 393 describes the examination of the crystal growth according to a modified Bridgman method of undoped large crystals of LaCl3 having high quality and without defects. These large crystals of LaCl3 can only be produced by removing water and oxygen during the production. But this is not a simple undertaking, because LaCl3 has hygroscopic properties and during the process of crystal growth contamination may take place. At first, the raw material is heated in a nitrogen atmosphere to 235° C. for 10 h, to remove humidity and crystallized water. Subsequently, the dried powder is pressed and the crucible is sealed completely so that no contamination takes place. According to this document, defects in the crystal are avoided and crystals of LaCl3 having high quality are obtained. However, the method is time-consuming and expensive.
US 2007/0241284 A1 discloses inorganic scintillation materials having the formula AnLnpX(3*p+n), wherein Ln represents one or more rare earth metals, X represents one or more halogen atoms, selected from F, Cl, Br or I, and A represents one or more alkali metals, such as K, Li, Na, Rb or Cs. Here, the inorganic scintillation material is stoichiometric. This scintillation material has a very low nuclear background noise and therefore it can be used in particular as a scintillator detector in the field of nuclear medicine, physics, chemistry and oil research and for the detection of hazardous or illegal materials.
U.S. Pat. No. 7,084,403 B2 describes monocrystalline or polycrystalline scintillation materials containing an activator such as e.g. Ce3+. The described scintillation material comprises lanthanum halogenides consisting either of a mixture of at least two halogenides, such as LaCl3 and LaBr3, or only of Lal3. Lal3 should essentially be free of LaOl2. All materials are stoichiometric.
U.S. Pat. No. 7,067,816 B2 describes inorganic scintillation materials having a composition of M1−xCexBr3, wherein M is selected from lanthanoids or lanthanoid mixtures of the group La, Gd and Y. X defines the molar ratio of M to cerium, wherein X is higher than 1% by mol and lower than 100% by mol. The scintillation material is stoichiometric. However, impurities which may occur during the production of the scintillation material may be present. These impurities are already present in the raw material in an amount of lower than 0.1% or even lower than 0.01%. These scintillation materials are used as detectors in industry and medicine or in the oil industry.
All scintillation materials known in prior art are stoichiometric. The term “stoichiometric” should mean the same as “in stoichiometric ratio”, i.e. the ratio of anion to cation is stoichiometric. Deviations from the stoichiometry usually lead to an increased density of defects which results in deteriorated properties of the scintillation material.